Tetrabromobisphenol-A hereafter referred to as TBBA is one of the most widely used and largest selling brominated flame-retardants in the world. It is used extensively to provide flame retardency for styrenic thermoplastics and for some thermoset resins. The major markets for flame-retardants are the electrical, electronic appliances, automotives, textiles and furniture industry. The high quality TBBA is mandatory for flame-retardant polymers and plastics to be used in the electronics industry.
There are several known methods for the manufacture of TBBA, most of them covered in patents. Bromination of bisphenol A (BA) is an essential step in all the methods employed to obtain TBBA.
Bromination of BA is conventionally carried out using molecular bromine for the manufacture of TBBA. Since this involves electrophilic bromination, this method generates one mole of hydrogen bromide as an effluent per every mole of molecular bromine consumed. The generated hydrogen bromide on interaction with methanol gives methyl bromide, a widely used fumigant. Therefore, the earlier plants have integrated approach to fulfill the twin objectives of production of TBBA on interaction with molecular bromine and of methyl bromide from methanol employing generated hydrogen bromide. Methyl bromide, a fumigant, which is being banned, necessitates the development of an alternative process to use hydrogen bromide generated in the bromination of BA. In this connection, the function of haloperoxidase enzymes to oxidise the nucleophilic Br to the electrophilic Br.sup.+, observed in nature, inspired us to explore the possibility of oxidising the nucleophile, Br, of the hydrogen bromide into the electrophilic bromine in order to use liberated HBr in the reaction of bromination of BA for the bromination of BA. Herein, we describe a heterogeneous catalyst, layered double hydroxides exchanged with tungstate that catalyses oxidative bromination in a selective manner. The low cost of the catalyst, reusability for several times and its ability to utilise the two atoms of the elemental bromine, raise the prospect of being successful in developing a clean and efficient industrial route to brominated chemicals and drugs.
Reference may be made to a U.S. Pat. No. 3,536,302, wherein TBBA is formed by reacting bisphenol-A and bromine in methanol. Thus, for the production of tetrabromobisphenol-A, equivalent amounts of HBr are generated. The HBr in turn reacts with the methanol solvent to produce the methyl bromide as a co-product. The drawbacks in the above process are the formation of methyl bromide, which is going to be a banned chemical and that the recovery and reuse of hydrogen bromide is cumbersome.
Reference may be made to a Japanese patent 77034620 B4 77/09/05 and U.S. Pat. Nos. 3,929,907; 4,180,684; 5,068,463, wherein the bisphenol-A is brominated in a biphase system comprising of water, water immiscible halogenated organic compound and an oxidant. The oxidant oxidizes the HBr to Br.sub.2, which in turn is then available to brominate more bisphenol-A and its under-brominated species. The disadvantages of these processes are longer reaction times and the high expense of handling. In addition, the cooling of the solution to recover tetrabromobisphenol-A entails additional expenditure and process time.
Reference may be made to Japanese patent 1979-55538, May 2, 1979, wherein TBBA was prepared by the bromination of bisphenol-A in the presence of organic solvents and aqueous solutions and an improvement in the product separation was done by incorporating an active surface agent at the end of the reaction, to cause the separation of the emulsion into a distinct phase. The drawback in the above process is that the product is of inferior quality.
Reference may be made to U.S. Pat. Nos. 4,990,321; 5,008,469; 5,059,726, and 5,138,103 wherein bisphenol-A is brominated at a low temperature, 0.degree. to 20.degree. C., in the presence of a methanol solvent and a specified amount of water. The amount of water used, however, is not so large so as to cause the precipitation of the tetrabromobisphenol-A from the reaction mass. Additional water for that purpose is added at the end of the reaction. The drawbacks with these processes are that they use a fairly long aging or cooking period after the reactants have all been fed and require an additional process time for the final precipitation of tetrabromobisphenol-A via the last water addition.
Reference may be made to a U.S. Pat. No. 6,002,050 wherein bisphenol-A saturated with TBBA is brominated in the presence of water, water miscible solvent containing H.sub.2 O.sub.2 and 1-20 wt. % of acid at a relatively high temperature. The drawbacks with this process are high temperature, long reaction times, presence of large amount of water and formation of small amounts of methyl bromide.
As long as there is a viable market for methyl bromide, the processes have been found to be commercially attractive. However, it is now being proposed, on an international level, that the use of methyl bromide as a fumigant be prohibited. Since the fumigant market is the main market for methyl bromide, a need is apparent for tetrabromobisphenol-A processes which do not co-produce a substantial amount of methyl bromide. This is a difficult task because, to be commercially successful, such processes will be required to economically produce tetrabromobisphenol-A without the benefit of the revenue realized from the sale of the co-product methyl bromide.
Obviously, different approaches have been employed to prepare TBBA. Our invention relates to the use of a heterogeneous catalyst, layered double hydroxides exchanged with tungstate for the preparation of TBBA.